What's where on Mauna Kea

Here at UCO/Lick Observatory we build instruments. Originally we built
for the telescopes at Lick Observatory on Mt. Hamilton (east of San
Jose, California). Lately we added the UCO part to our name, and
we have also been building instruments for the telescopes at Keck
Observatory on Mauna Kea (the big island of Hawaii).
Having spent some effort determining
what is where on Mt. Hamilton, it became time
to do the same for Mauna Kea.

The 7.5 minute USGS topographic quadrangle for Mauna Kea is
available online. Unfortunately,
many of the observatories have been built subsequent to the most
recent update some 20 years ago.

The observatories on Mauna Kea do not share the geodetic favor that
was so helpful on Mt. Hamilton. The environment on Mauna Kea is not
conducive to casual geodetic occupation, and the few surrounding peaks
of equal size are similarly difficult. Nevertheless, the NGS provides a wonderful
resource that permits the inspection of the data sheets for all
the reference marks in the US, including those on the
Mauna Kea summit.

Putting these bits of information together creates this map. Each of the control points from the
NGS data sheets is marked. The approximate location of most of the
telescopes are also marked. The VLBA antenna is conspicuously absent,
but it is located off the image approx. 2km south-southeast of the
summit at the 12200 foot level.

From scaling this map I estimate that the position of Keck I is
W 155:28:28.8
N 19:49:35.0
using the 1983 North American Datum.
Because Keck is not marked on the quadrangle, these positions could
be wrong by as much as an arcsecond.
A much better set of positions for most everything is at the
IFA web site.

Several aspects of these data need to be emphasized.

Note that the only NGS reference marks are on the N-S ridge
with the original UH telescopes. There are no marks along
the E-W ridge leading to Keck and Subaru.

The long/lat grid marks on the USGS topo are for the 1927 North American
Datum. The 1927 NAD is based on the 1866 Clarke ellipsoid and tied
to the earth at Meade Ranch, Kansas; it is a very bad fit to the
geoid in the vicinity of Hawaii.
The marks for the 1983 North American Datum would be 291 meters
west and 341 meters north of the marks shown.

The 1983 NAD uses a different ellipsoid than the system in use by the
GPS satellites, and there is active discussion going on about replacing
the 1983 NAD with yet a better datum.

The USGS topo has been rotated to align with north on the Universal
Transverse Mercator (UTM) grid (the difference between geodetic north
and UTM grid north is given on the data sheets).

The USGS topo shows only those
domes which were present in 1982. The dome shown at the current location
of the Gemini is actually a previous UH telescope, now destroyed.

The construction of the CFHT may have obliterated the control point
with NGS PID TU2308.

It is not clear that there have ever been any gravity measurements
on Mauna Kea, nor any transit observations of the sort that would
determine the deflection of the vertical. This means that there is
no way to get an accurate value for the astronomical long/lat
of Keck.

There must be other survey data not in the NGS database, for the VLBA
antenna would not be able to obtain fringes if its position were not
known within a few meters.

A significantly more reliable scaling could be performed by using site
maps from the construction of the new domes. In more hospitable
locations the sorts of maps which show the layout of underground
utilities and roadways are usually tied to survey marks. These are
typically found in blueprint rooms and on file at county permit
offices.
Steve Allen <sla@ucolick.org> UCO/Lick ObservatoryUniversity of California
Santa Cruz, CA 95064